This application requests an Administrative Supplement to Grant R21 AI087472 ?Engineered envelope glycoprotein trimers for HIV-1 vaccine immunogens?, a grant awarded in 2009, and now in a second no-cost extension. As a result of power outages during Hurricane Sandy, sera collected from rabbits immunized with two modified gp140 proteins were irretrievably lost. While preliminary measurements of anti-Env antibody titers in the rabbit serum suggest promising outcomes for this project, we are unable to complete our analysis to assess the extent and breadth of the serum neutralizing activity. The supplemental funds will enable us to repeat our rabbit immunization experiments. These immunogenicity studies will provide a platform for new R01 research, intended to design and develop HIV-1 Env immunogens that present key quaternary epitopes in a stable state capable of eliciting broadly neutralizing HIV-1 antibodies in humans. Experiment 1: To produce and characterize homogeneous preparations of biochemically stabilized HIV-1 gp140 trimers. Experiment 2: To conduct immunogenicity studies in rabbits to determine whether stable native gp140 trimers can improve neutralizing antibody responses. Our overall goal is to understand how specific gp41-gp41 interactions modulate the trimerization and stability of the prefusion Env complex and to apply this knowledge to design native Env-trimer mimics with enhanced immunogenicity of neutralizing epitopes. To do this, we will express and purify to homogenicity modified HIV-1 JR-FL gp140 trimers, and evaluate their immunogenicity in rabbits. These interlinked experiments are intended to generate proof-of-concept information in the rabbit model that can be used to address our overarching hypothesis that specific cooperative inter-subunit interactions in native Env trimer lead to its inherent instability and can be engineered to enhance immunogenicity. The with-cost extension of support would significantly overcome losses of materials incurred by Sandy that we need desperately in order to fulfill the central objective of the parent grant.